Jotun Yachting

The osmosis process

Osmosis is the natural process of liquids to equal the differences in pressure between the humidity content of the hull and the sea water. The gelcoat is not a totally waterproof barrier, so humidity will penetrate through the gelcoat down to the fibre glass laminate. 

Other factors that contribute to the development of osmosis, are water and moisture in the bilges. This moisture will also penetrate the laminate from inside, thus enabling osmosis to occur. The speed of the osmosis process and the damage it may cause is depending on several factors, such as production speed, glass fibre quality, water temperature etc. Osmosis is a potential problem for gelcoat hulls in the same way corrosion threaten steel and aluminium hulls. To prevent osmosis, it is important to apply a correct paint system as soon as possible.

How does osmosis occur?

Osmosis is the process where water is transported through a film, e.g gelcoat, from an area with high concentration of water to an area with low concentration of water in order to achieve equal water concentration in the two areas. These areas may be in the polyester binder itself (yellow area), in the glass fibre laminate (grey area) or between the glass fibre laminate and the gelcoat (red area). After some time, both sides of the gelcoat have an equal concentration of water, which will create blisters with an hydraulic pressure behind the gelcoat.

How to detect and treat a hull affected by osmosis

It is not always easy to detect osmosis in a hull. Only when blistering can be seen, it is easy to check if the hull has osmosis. If the blistering, blisters (usually of different sizes) contain a brown coloured liquid with a vinegar odour, it is reason to believe that the hull has osmosis and an osmosis treatment must be initiated. If blisters are dry and hard, they are the consequence of air or solvent retention during the painting works, which has no effect on the fibre glass. 

If blisters are not visible, this does not mean that hull is osmosis free: as we seen in the ”Osmosis process” explanation, the process requires a certain time before blisters are likely to appear. Furthermore preventive treatments will not stop the osmotic process, so it is advisable to get the hull inspected by a professional who can confirm any osmosis status or potential. If so, a repair treatment should be initiated as soon as possible to avoid the progression of osmosis. 

If a full osmosis treatment has to be carried out, the gelcoat must be removed completely, the hull must be left to dry and the Jotun Yachting Osmoshell system should be applied to the fibre glass before any primer or antifouling. The most important point is to replace the gelcoat with the correct thickness with epoxy products which are much more waterproof than gelcoat.